The previously formulated rate-type aging creep law based on Maxwell chain is generalized to variable humidity and is calibrated by extensive comparisons with test data from the literature. The main object of attention is the Pickett effect, i.e., the apparent increase in creep due to drying simultaneous with loading. This effect is shown to have four sources, in their decreasing order of importance: (1) stress-induced shrinkage, (2) tensile strain softening due to progressive cracking, (3) irreversibility of unloading contraction after tensile strainsoftening, and (4) increase of material stiffness due to aging (hydration). The model, which is a special case of a previously advanced thermodynamic theory, depends on only one hypothesis about the microscopic physical mechanism of creep: The creep rate depends on the magnitude of the flux of microdiffusion of water between the macropores (capillary pores) and the micropores in the cement gel. By assuming this microdiffusion to be infinitely fast, the effect is reduced to a dependence of creep viscosities on the time rate of pore humidity, and this is further shown to be equivalent to stress-induced shrinkage, in which the shrinkage coefficient defining the ratio of the increments of shrinkage strain and pore relative humidity depends on stress. In three dimensions, the shrinkage coefficient thus becomes a tensor. For thermodynamic reasons, there must also exist stress-induced thermal expansion. Although tensile cracking is found to make significant contribution to the Pickett effect, it is far from sufficient to explain in fully. The theory agrees with test data on basic creep, creep of specimens with reduced water content at hygral equilibrium (predried), shrinkage, swelling, and creep at drying under compression, tension, or bending. The strainsoftening model used for tensile cracking is the same as that used previously to fit test data from fracture tests, direct tensile tests, and deflection tests of reinforced beams.
ASJC Scopus subject areas
- Civil and Structural Engineering
- Building and Construction
- General Materials Science
- Mechanics of Materials